In case you missed it - Tamping essential for good track alignment

Track tamping machines play a vital role in the maintenance of track. They are complex and expensive railway maintenance machines and can be found all across the world’s railways. Yet often little is known about them. For instance, what is their purpose and how do they work?

Firstly the reason tampers exist – railway track settles with the passage of traffic. This would not be a problem except that track doesn’t settle evenly, for many and various reasons, resulting in an uneven track formation. Trains require good alignment of the top of the rails. If not maintained correctly, rough riding and even derailments will occur. The faster the train travels along the track, the more important it is to maintain that good alignment.

The development of tamping

Years ago, alignment of track was manually maintained and required whole teams of men using sighting board levels and small tin cans full of 1⁄2” stone. They would manually align track with bars to improve lateral position using skill, string lines and sighting boards.

Track top alignment was achieved either by using levelling boards or by measuring dips to be corrected. Men would open out affected beds, jack the track (often in between trains) and add quantities of 1⁄2” stone skilfully placed from shovels. The amount of stone required was measured out from small tin cans at the rate of one can = 1/8” of lift.

Over the years, the then British Railways progressively phased out manpower, replacing it with tamping machines. We are now at the point today where tamping machines are used almost exclusively to achieve good top and line of the track. They work extremely well and are efficient and accurate.

The tamper adopts a different technique – it consolidates ballast beneath each sleeper creating pyramids of consolidated stone.

To do this, tamping tools are driven into the ballast, drawn together whilst vibrating at an appropriate frequency to fluidise the ballast. During the period the tines are driven into the ground, hydraulic jacks lift and align the rails.

Early tamping machines would simply pack the track but, as technology evolved, so did the tampers. There are several different types of tamper machines; plain line track tamping machines and points and crossing tamping machines, some with 12 tools for use on tracks with conductor rails, others with 16 tools where there is no conductor rail. Then there are also 32 tools to tamp two sleepers at a time and 48 tools to tamp three sleepers at a time. All are diesel powered and can travel across the country at speeds of up to 60mph (100km/h) – this same engine is used to power the track tamping equipment.

Balfour Beatty has a seven-year contract to supply the Network Rail National Supply Chain (NSC) with on-track machines including tampers, and has been providing track alignment services in the South East, Anglia and Wessex routes for over 25 years. This section of the railway is the most densely used by the travelling public, with people travelling or commuting in and out of London. Consequently, ensuring the track is perfectly aligned comes with great challenges and responsibility. Managing risk and safety is pivotal in all operational activity and, to assist with this, Balfour Beatty utilises a 24-hour control centre service which logs real-time data from live sites and provides activity reports on request to Network Rail and key stakeholders.

Why tamp?

Tampers fix track problems caused by the passage of traffic. The greater the tonnage often means the larger the movement of the track and the more tamping intervention is required. During a typical maintenance shift, a tamping machine may maintain around a mile of track. These days, to maximise the benefit of the tamping machine intervention, work can be focused into very short lengths of track to correct specific issues.

Network Rail operates various systems to understand the condition of its track assets; most obvious is the high-speed track recording train. This travels the entire national network recording track quality as it goes. The output is fed down to the local track managers who monitor trends in track degradation and, from that trend analysis, develop their maintenance plan, which also includes tamping.

Balfour Beatty’s Engineering and Technology Solutions business, in fact, supports Network Rail with collecting this data through innovative track measurement equipment that is deployed across the country and feeds into the National Gauging Database. The maintenance and tamping plan results in a tamping machine arriving at the worksite to correct the recorded track errors.

Tamping explained

All modern high-performance tamping machines have a measurement system to enable them to understand where faults exist on the track. This measurement system requires three trolleys – one at the front of the machine, one in the centre (known as the measuring trolley) and one to the rear. A straight line is created between these three trolleys in the form of a beam of light or a tension wire. At the measuring trolley, any misalignment – horizontal or vertical – is recorded as an electrical signal. These signals are fed into the tamper’s control system that directs the working units to adjust the track.

There are a number of ways in which a tamper works. Simple smoothing is where the tamper surveys and adjusts the track using errors measured only between the front and rear trolleys. The tamping machine can only see the errors between those two trolleys, therefore the machine can only improve track over that short distance. This can leave long-wavelength faults and is generally not used in the UK anymore.

The second method is more commonly used, and involves the tamper measuring the entire track length between ‘Point A’ and ‘Point B’, a distance which may be up to a mile in length. The machine’s guidance computer notes all the track errors and subsequently computes a track design using an algorithm to provide the best possible horizontal and vertical alignment through the points.

If, during the measurement process, there is a requirement not to move the track, for instance where a point passes close through a bridge, tunnel or station platform, the operator puts fixed points into the programme and that tells the computer to hold the track position at those points.

Once the computer has developed its programme the operator and inspector check it and, once satisfied, commit the program to the system, position the machine correctly and commence tamping. The benefit of this method of working is it has the effect of extending the length of the tamping machines guidance system to the full length of the site, ensuring there are no long-wavelength track faults.

The third method of working is usually known as the geometry method. This is most often used in track renewal on the West Coast main line, where the as-built design is available, especially around areas of switches and crossings. A design for the track to be tamped is developed off-site by technical staff, and is input to the tamper’s computer to allow it to understand what the finished track should look like. The machine makes a measurement run of the site and the output is the difference between the design and the track as it is before correction. A correction file is generated which informs the machine where and how much track to deploy.

New software has been developed that allows the tamping machine to repair discreet errors on a single rail. As described earlier, tamping machines use measuring trollies to understand where the track is and what errors may be in that track. The new piece of software allows the machine to correct discreet errors on single rails without the need to maintain hundreds of metres of track. This is particularly useful on high-speed railways, in particular High Speed 1. Using this innovation, tamping machines can rectify 20-30 metre faults without disturbing the other rail.

Youthful and versatile

Within its fleet, Balfour Beatty owns 17 tampers, which include eight Compact Plasser and Theurer machines, six Matisa Tampers and two Plasser Unimats. Of these tampers, 16 are under contract to Network Rail. The newest machines, the Matisas, were built in 2010 with an investment of approximately £20 million
for the six machines. The asset life of a tamper machine is 15-20 years and. with the exception of the Network Rail High Output fleet, Balfour Beatty has the most modern fleet of tampers in the UK.

All machines are fitted with the necessary equipment to work on the main line, just like the locomotives and multiple units that run on Network Rail tracks. To that end, they are fitted with Automatic Warning System (AWS), GSM-R, a Train Protection Warning System (TPWS) and On-Train Monitoring Recorders (OTMR). The whole fleet is also fitted with Track Circuit Actuators (TCA), to enable detection on track circuits when travelling on the network.

Balfour Beatty owns the only two Matisa B66UCs in the UK. These are continuous-action universal tamping machines, which can tamp plain line as well as switches and crossings. When in plain-line mode, these machines can operate cyclically or continuously.

Tampers have traditionally operated cyclically, that is why they stop over the sleeper/s to be tamped, complete the process and move onto the next sleeper. This requires the machine to stop and accelerate at every sleeper (or pair or three sleepers depending upon the type of tamper). When in continuous action mode, the body of the machine moves ahead continuously but the working units stop start and accelerate forward, saving fuel and improving the operator experience.

Tamping through switches and crossings is a slow process as the track geometry is constantly changing as the machine moves through the switch. Typically, it takes around 40 minutes to complete a single switch (lead), depending upon the cabling and condition.

Balfour Beatty has a significant footprint in the South, with depots at Woking, Colchester and Hither Green and further access to depots in Ashford, Romford, Eastleigh, Three Bridges and a Midlands depot in Sandiacre, Derbyshire. All of the fleet has been adapted to work within third rail environments, so Balfour Beatty machines can be easily deployed to operate across the network.

Track renewal and maintenance play a vital role in ensuring that the rail network operates safely and reliably. The function of tampers in maintaining track alignment is equally as important. Balfour Beatty has deep route knowledge, expertise and capability in maintaining the nation’s railways and is investing in research and development to further improve the infrastructure and help reduce the cost of track maintenance.

Written by Geoff Brown, engineering development manager, Balfour Beatty.